Lead with MRI compatible design features

ABSTRACT

Implantable medical leads with magnetic shielding and methods of shielding implantable leads from magnetic fields during medical procedures such as magnetic resonance imaging (MRI) are disclosed. An exemplary implantable medical lead includes a helically coiled inner electrode conductor wire, a helically coiled outer electrode conductor wire disposed radially about the inner electrode conductor wire, and at least one layer of insulation that electrically isolates the inner and outer electrode conductor wires. The inner electrode conductor wire can have a hollowed, multifilar configuration including six or more co-radially wound wire filars. The outer electrode conductor wire is electrically isolated from the inner electrode conductor wire, and may have either a single or double filar configuration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/364,181, filed on Feb. 2, 2009, which claims priority under 35 U.S.C.§119 to U.S. Provisional Application No. 61/026,661, filed on Feb. 6,2008, entitled “Lead With MRI Compatible Design Features,” each of whichare incorporated herein by reference in their entirety for all purposes.

TECHNICAL HELD

The present invention relates to medical devices and the simultaneousdelivery of diagnostic and therapeutic treatments. More specifically,the present invention relates to implantable medical leads with magneticshielding and methods of shielding such leads from magnetic fieldsduring medical procedures such as magnetic resonance imaging (MRI).

BACKGROUND

Magnetic resonance imaging (MRI) is a non-invasive imaging method thatutilizes nuclear magnetic resonance techniques to render images within apatient's body. Typically, MRI systems employ the use of a magnetic coilhaving a magnetic field strength of between about 0.2 to 3 Teslas.During the procedure, the body tissue is briefly exposed to RF pulses ofelectromagnetic energy in a plane perpendicular to the magnetic field.The resultant electromagnetic energy from these pulses can be used toimage the body tissue by measuring the relaxation properties of theexcited atomic nuclei in the tissue.

During imaging, the electromagnetic radiation produced by the MRI systemmay be picked up by implantable device leads used in implantable medicaldevices such as pacemakers or cardiac defibrillators. This energy may betransferred through the lead to the electrode in contact with thetissue, which may lead to elevated temperatures at the point of contact.The degree of tissue heating is typically related to factors such as thelength of the lead, the conductivity or impedance of the lead, and thesurface area of the lead electrodes. Exposure to a magnetic field mayalso induce an undesired voltage on the lead.

SUMMARY

The present invention relates to implantable medical leads with magneticshielding and methods of shielding implantable leads from magneticfields during medical procedures such as magnetic resonance imaging(MRI). An illustrative medical device includes a pulse generator and alead having a helically coned inner electrode conductor wire, ahelically coned outer electrode conductor wire, and one or moreinsulation layers. The inner electrode conductor wire has a hollowed,multifilar configuration including six or more co-radially wound wirefilars. The outer electrode conductor wire is electrically isolated fromthe inner electrode conductor wire, and has either a single filar ordouble filar configuration with a relatively high inductance that isadapted to dissipate electromagnetic energy received by the lead duringa magnetic resonance procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an illustrative medical device having alead implanted within the body of a patient;

FIG. 2 is a schematic view showing a simplified equivalence circuit forthe lead of FIG. 1;

FIG. 3 is a view showing the interior construction of the lead of FIG. 1in accordance with an exemplary embodiment; and

FIG. 4 is a cross-sectional view showing the lead along line 4-4 in FIG.3.

While the invention is amenable to various modifications and alternativeforms, specific embodiments have been shown by way of example in thedrawings and are described in detail below. The intention, however, isnot to limit the invention to the particular embodiments described. Onthe contrary, the invention is intended to cover all modifications,equivalents, and alternatives falling within the scope of the inventionas defined by the appended claims.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of an illustrative medical device 12 havingwith a lead implanted within the body of a patient. In the illustrativeembodiment depicted, the medical device 12 comprises a pulse generatorimplanted within the body. The pulse generator 12 is coupled to a lead14 inserted into the patient's heart 16. The heart 16 includes a rightatrium 18, a right ventricle 20, a left atrium 22, and a left ventricle24. The pulse generator 12 can be implanted subcutaneously within thebody, typically at a location such as in the patient's chest or abdomen,although other implantation locations are possible.

A proximal section 26 of the lead 14 can be coupled to or formedintegrally with the pulse generator 12. A distal section 28 of the lead14, in turn, can be implanted at a desired location in or near the heart16 such as in the right ventricle 20, as shown. In use, one or moreelectrodes 30 on the distal section 28 of the lead 14 may providetherapy to the patient in the form of an electrical current to the heart16. In certain embodiments, for example, the electrode(s) 30 may beprovided as part of a cardiac lead 14 used to treat bradycardia,tachycardia, or other cardiac arrhythmias.

Although the illustrative embodiment depicts only a single lead 14inserted into the patient's heart 16, in other embodiments multipleleads can be utilized so as to electrically stimulate other areas of theheart 16. In some embodiments, for example, the distal section of asecond lead (not shown) may be implanted in the right atrium 18. Inaddition, or in lieu, another lead may be implanted in or near the leftside of the heart 16 (e.g., in the coronary veins) to stimulate the leftside of the heart 16. Other types of leads such as epicardial leads mayalso be utilized in addition to, or in lieu of, the lead 14 depicted inFIG. 1.

During operation, the lead 14 can be configured to convey electricalsignals between the pulse generator 12 and the heart 16. For example, inthose embodiments where the pulse generator 12 is a pacemaker, the lead14 can be utilized to deliver electrical therapeutic stimulus for pacingthe heart 16. For example, in the treatment of bradycardia ortachycardia, the pulse generator 12 can be utilized to deliverelectrical stimulus in the form of pacing pulses to the heart 16. Inother embodiments in which the pulse generator 12 is an implantablecardiac defibrillator, the lead 14 can be utilized to deliver electricshocks to the heart 16 in response to an event such as a heart attack orarrhythmia. In some embodiments, the pulse generator 12 includes bothpacing and defibrillation capabilities.

When the pulse generator 12 is subjected to a magnetic field from an MRIscanner or other external magnetic source, electromagnetic radiation isproduced within the body that can be picked up by the lead 14 andtransferred to the lead electrode(s) 30 in contact with the body tissue.This electromagnetic radiation can cause heating at the interface of thelead electrode(s) 30 and body tissue, and can interfere with thetherapeutic electrical currents transmitted by the pulse generator 12through the lead 14.

FIG. 2 is a schematic view showing a simplified equivalence circuit 32for the lead 14 of FIG. 1, representing the RF energy picked up on thelead 14 from RF electromagnetic energy produced by an MRI scanner. Asshown in FIG. 2, Vi 34 in the circuit 32 represents an equivalent sourceof energy picked up by the lead 14 from the MRI scanner. During magneticresonance imaging, the length of the lead 14 functions similar to anantenna, receiving the RF energy that is transmitted into the body fromthe MRI scanner. Voltage (Vi) 34 in FIG. 2 may represent, for example,the resultant voltage received by the lead 14 from the RF energy. The RFenergy picked up by the lead 14 may result, for example, from therotating RF magnetic field produced by an MRI scanner, which generatesan electric field in the plane perpendicular to the rotating magneticfield vector in conductive tissues. The tangential components of theseelectric fields along the length of the lead 14 couple to the lead 14.The voltage (Vi) 34 is thus equal to the integration of the tangentialelectric field (i.e., the line integral of the electric field) along thelength of the lead 14.

The ZI parameter 36 in the circuit 32 represents the equivalentImpedance exhibited by the lead 14 at the RF frequency of the MRIscanner. The impedance value ZI 36 may represent, for example, theinductance or the equivalent impedance resulting from the parallelinductance and the coil turn by turn capacitance exhibited by the lead14 at an RF frequency of 64 MHz for a 1.5 Tesla MRI scanner, or at an RFfrequency of 128 MHz for a 3 Tesla MRI scanner. The impedance ZI of thelead 14 is a complex quantity having a real part (i.e., resistance) andan imaginary part (i.e., reactance).

Zb 38 in the circuit 32 may represent the impedance of the body tissueat the point of lead contact. Zc 40, in turn, may represent thecapacitive coupling of the lead 14 to surrounding body tissue along thelength of the lead 14, which may provide a path for the high frequencycurrent (energy) to leak into the surrounding tissue at the RF frequencyof the MRI scanner. Minimizing the absorbed energy (represented bysource Vi 34) reduces the energy that is transferred to the body tissueat the point of lead contact with the body tissue.

As can be further seen in FIG. 2, the lead 14 has some amount of leakage40 into the surrounding tissue at the RF frequency of the MRI scanner.As further indicated by 38, there is also an impedance at the point ofcontact of the lead electrode(s) 30 to the surrounding body tissuewithin the heart 16. The resulting voltage Vb delivered to the bodytissue may be related by the following formula:Vb=Vi Zbe/(Zbe+ZI), where Zbe=Zb in parallel with Zc.The temperature at the tip of the lead 14 where contact is typicallymade to the surrounding tissue is related in part to the powerdissipated at 38 (i.e., at “Zb”), which, in turn, is related to thesquare of Vb. To minimize temperature rises resulting from the powerdissipated at 38, it is thus desirable to minimize Vi (34) and Zc (40)while also maximizing the impedance ZI (36) of the lead 14. In someembodiments, the impedance ZI (36) of the lead 14 can be increased atthe RF frequency of the MRI scanner, which aids in reducing the energydissipated into the surrounding body tissue at the point of contact 38.

In some embodiments, the impedance of the lead 14 can be increased byadding inductance to the lead 14 and/or by a suitable constructiontechnique. For example, the inductance of the lead 14 can be increasedby increasing the diameter of the conductor coil(s) and/or by decreasingthe pitch of the conductor coil(s) used to supply electrical energy tothe electrode(s) 30. Decreasing the coil pitch may result in increasingcapacitance between successive turns of the coil (i.e., coil turn byturn capacitance). The parallel combination of inductance (from thehelical shape of the coil) and the turn by turn capacitance constitutesa resonance circuit. For a helically coiled lead construction, if theresonance frequency of the lead is above the RF frequency of the MRI,then the helical coil acts as an inductor. For an inductor, increasingthe cross section of the coil area and/or reducing the coil pitchincreases the inductance and, as a result, increases the impedance ofthe lead 14.

Similar to an antenna, the energy pickup from a lead is related to itsresonance length with respect to the wavelength of the frequency ofinterest. For example, for a dipole antenna, the antenna is consideredtuned, or at resonance, when the antenna length is half the wavelengthor an integer multiple of the wavelength. At resonance lengths, theenergy pickup of the antenna is maximized. In a similar manner, and insome embodiments, the lead 14 can be detuned so as to prevent resonancewithin the lead 14, and thus minimize the voltage Vi. For theillustrative embodiment shown in FIG. 1, for example, the lead 14functions as an antenna having a resonance frequency at lengthL=integer×λ/2. In some embodiments, the length of the lead 14 and/or theconstruction parameters of the lead 14 affecting the wavelength can bechosen so as to avoid resonance within the lead 14.

In some embodiments, in addition to detuning the length of the lead 14with respect to the wavelength of the MRI induced RF energy, shieldingcan also be added to the lead 14 to further reduce the amount ofelectromagnetic energy picked up from the lead 14. For example, theenergy picked up from the shielding can be coupled to the patient's bodyalong the length of the lead 14, preventing the energy from coupling tothe lead tip. The transfer of intercepted energy by the shielding alongthe length of the shielding/lead can also be inhibited by dissipatingthe energy as resistive loss, using resistive material for the shieldingconstruction.

FIG. 3 is a view showing the interior construction of the lead 14 ofFIG. 1 in accordance with an exemplary embodiment. In the embodiment ofFIG. 3, the lead 14 includes an inner electrode conductor wire 42, anouter electrode conductor wire 44, and an insulation layer 46 disposedradially about the outer electrode conductor wire 44. The innerconductor wire 42 can have any number of different configurations knownin the art, including but not limited to, a coiled configuration, acable configuration, a straight wire configuration, or the like.

In the illustrative embodiment of FIG. 3, the inner conductor wire 42comprises a helically-shaped multifilar coil conductor wire having anumber of filar strands 48 that are tightly wound together to form aninner electrode used to deliver electrical stimulus energy through thelead 14. In one embodiment, for example, the inner conductor wire 42includes six or more filar strands 48 forming a helically-shapedconductor. In other embodiments, the inner conductor wire 42 can includea greater or lesser number of filar strands 48. In one embodiment, forexample, the inner conductor wire 42 may comprise twelve co-radiallywound filar strands 48. In some embodiments, each of the filar strands48 forming the inner conductor wire 42 can comprise a silver-filledMP35N wire having a silver content of about 10% to 28% bycross-sectional area.

In some embodiments, the inner conductor wire 42 has a hollowedconfiguration, including an interior lumen 50 extending through the wire42 and adapted to receive a stylet or guidewire that can be usedfacilitate implantation of the lead 14 within the body. In certainembodiments, the inner conductor wire 42 can be fabricated byco-radially winding a number of wire filars about a mandrel having adiameter that is slightly greater than the diameter of the stylet orguidewire to be inserted into the lumen 50. To improve the torquecharacteristics of the wire 42, the wire filars 48 can be tightly woundtogether during fabrication of the wire 42 such that no gaps or spacesexist between the filar strands 48.

As further shown in FIG. 3, and in some embodiments, the outer conductorwire 44 is coaxially disposed about the inner conductor wire 42 and hasa helically coiled configuration that extends along all or a portion ofthe length of the lead 14. In some embodiments, the outer conductor wire44 has a single-filar construction formed from a single wound wire. Inother embodiments, the outer conductor 44 has a multifilar constructionformed from multiple, co-radially wound wire filars. In one embodiment,for example, the outer conductor wire 44 has a double-filar constructionformed from two co-radially wound wire filars.

The outer conductor wire 44 can be spaced radially apart from the innerconductor wire 44, electrically isolating the outer conductor wire 44from the inner conductor wire 42. In some embodiments, for example, theouter conductor wire 44 is electrically isolated from the innerconductor wire 42 so that the lead 14 can function as a multipolar lead.In certain embodiments, a second layer of insulation 52 interposedbetween the inner conductor wire 42 and the outer conductor wire 44 isfurther used to electrically isolate the conductor wires 42,44 from eachother. In some embodiments, for example, the second layer of insulation52 may comprise a sheath made from silicon, polyurethane, or othersuitable polymeric material.

FIG. 4 is a cross-sectional view showing the lead 14 along line 4-4 inFIG. 3. As further shown in FIG. 4, and in some embodiments, the outerconductor wire 44 is formed from a small diameter wire to decrease theeffective pitch of the wire 44, which, in turn, increases the inductanceof the wire 44. In some embodiments, for example, the wire diameter D₁of the outer conductor wire 44 is in the range of between about 0.001 to0.006 inches, and more specifically, about 0.003 to 0.004 inches. Thewire diameter D₁ of the outer conductor wire 44 may be greater orlesser, however, depending on the type of lead employed, theconfiguration of the lead, as well as other factors. Due to therelatively small diameter D₁ of the outer conductor wire 44, a greaternumber of coil turns is present along the length of the lead 14 incomparison to more conventional leads with larger wire diameters, whichincreases the impedance of the conductor wire 44. This increasedimpedance aids in reducing the energy dissipated into the surroundingbody tissue at or near the lead electrode(s) 30.

The overall diameter D₂ of the outer conductor wire 44 can also beincreased to further increase the inductance of the wire 44. In someembodiments, for example, the overall diameter D₂ of the outer conductorwire 44 is in the range of between about 0.051 to 0.068 inches, and morespecifically, about 0.053 to 0.066 inches. The overall diameter of theouter conductor wire 44 may be greater or lesser, however, depending onthe type of lead employed, the configuration of the lead, as well asother factors. In some embodiments, the overall diameter of the lead 14is in the range of between about 3 to 7 Fr, and more specifically,between about 5 to 6 Fr.

In some embodiments, the outer conductor wire 44 is formed from adrawn-filled tube having an outer tubular layer of low-resistive metalor metal-alloy such as MP35N filled with an inner core of electricallyconductive material such as silver. Once filled and drawn, the tube isthen coiled into a helical shape and attached to the lead 14 usingconventional techniques know in the art. In one embodiment, the outerconductor wire 44 comprises a silver-filled MP35N wire having a silvercontent of about 28% by cross-sectional area. In use, the relatively lowresistance of the outer tubular metal or metal-alloy forming part of theouter conductor wire 44 can be used to offset the increased resistanceimparted to the wire 44 from using a smaller diameter wire, as discussedabove. In some embodiments, the material or materials forming the outerconductor wire 44 can also be selected so as to impart greaterflexibility to the wire 44.

The outer conductor wire 44 may be formed from a material or materialsdifferent than the inner conductor wire 42 in order to impart greaterresistance to the outer conductor wire 44 to aid in dissipating RFelectromagnetic energy received during an MRI procedure. In oneembodiment, for example, the wire filars forming the outer conductorwire 44 may comprise a silver-filled MP35N material having a silvercontent (by cross-sectional area) of about 28% whereas the wire Warsforming the inner conductor wire 42 may have a silver content (bycross-sectional area) lower than 28%.

As further shown in FIG. 4, and in some embodiments, the inner conductorwire 42 has a wire diameter D₃ of between about 0.001 to 0.004 inches,and more specifically, about 0.002 inches. In certain embodiments, theouter diameter D₄ of the inner conductor wire 42 is between about 0.020to 0.028 inches, and more specifically, between about 0.022 to 0.023inches. The dimensions of the inner conductor wire 42, including thewire diameter D₃ and outer diameter D₄ may vary, however.

By increasing the inductance of the lead 14, and in particular theinductance of the outer conductor wire 44, the lead 14 is configured todissipate RF electromagnetic energy received during a magnetic resonanceimaging procedure. This dissipation of electromagnetic energy results ina reduction in heating of body tissue at the location of theelectrode(s) 30. The increase in inductance of the lead 14 also reducesthe effects of the electromagnetic energy on the therapeutic electricalcurrent delivered through the lead 14, and in some cases, may permit thelead 14 to continue to provide therapy during the MRI procedure. In someembodiments, for example, the increase in inductance of the lead 14allows the lead 14 to function at normal device frequencies (e.g., 0.5Hz to 500 Hz) while acting as a poor antenna at MRI frequencies.

While the illustrative lead 14 is described with respect to a cardiaclead for use in providing pacing to a patient's heart 16, theconstruction of the lead 14 may also be applicable to other medicaldevices that operate in the presence of electromagnetic fields. Forexample, the construction of the lead 14, including the inner and outerconductor wires 42,44, may be used in neural leads adapted for use inneurological applications that utilize MRI imaging.

Various modifications and additions can be made to the exemplaryembodiments discussed without departing from the scope of the presentinvention. For example, while the embodiments described above refer toparticular features, the scope of this invention also includesembodiments having different combinations of features and embodimentsthat do not include all of the described features. Accordingly, thescope of the present invention is intended to embrace all suchalternatives, modifications, and variations as fall within the scope ofthe claims, together with all equivalents thereof.

What is claimed is:
 1. An implantable medical lead, comprising: ahelically-shaped inner electrode conductor wire disposed within aninterior lumen of the lead, the inner electrode conductor wirecomprising a plurality of co-radially wound wire filars wound to have afirst pitch, each wire filar of the inner electrode conductor wirecomprising a drawn-filled tube filled with an inner core of a first typeof material having a first resistance, the inner electrode conductorwire having a first outer diameter; a helically-shaped outer electrodeconductor wire disposed about and spaced radially apart from the innerelectrode conductor wire, the outer electrode conductor wire comprisinga plurality of co-radially wound wire filars wound to have a secondpitch that is less than the first pitch, each wire filar of the outerelectrode conductor wire comprising a drawn-filled tube filled with aninner core of a second type of material having a second resistance thatis less than the first resistance, the outer electrode conductor wirehaving a second outer diameter that is greater than the first outerdiameter; at least one insulation layer disposed radially about theinner electrode conductor wire; and wherein the outer electrodeconductor wire is configured to have an inductance sufficient todissipate electromagnetic energy received on the lead from magneticresonance imaging procedures, the inductance of the outer electrodeconductor wire based at least in part on the second pitch and the secondouter diameter.
 2. The medical lead of claim 1, wherein each wire filarof the inner electrode conductor wire has a wire diameter of betweenabout 0.001 inches to 0.004 inches.
 3. The medical lead of claim 1,wherein the first outer diameter is between about 0.020 inches to 0.028inches.
 4. The medical lead of claim 1, wherein the first type ofmaterial has a silver content by cross-sectional area lower than 28% andthe second type of material has a silver content by cross-sectional areaof about 28%.
 5. The medical lead of claim 1, wherein the outerelectrode conductor wire has a wire diameter of between about 0.001inches to 0.006 inches.
 6. The medical lead of claim 1, wherein thefirst outer diameter is between about 0.051 inches to 0.068 inches. 7.The medical lead of claim 1, wherein a wire diameter of the outerelectrode conductor wire is different than a wire diameter of the innerelectrode conductor wire.
 8. The medical lead of claim 1, wherein the atleast one insulation layer comprises: a first insulation layer disposedabout the outer electrode conductor wire; and a second insulation layerinterposed between the outer electrode conductor wire and the innerelectrode conductor wire.
 9. An implantable medical lead, comprising: ahelically-shaped inner electrode conductor wire disposed within aninterior lumen of the lead, the inner electrode conductor wirecomprising at least one radially wound wire filar, each wire filar ofthe at least one wire filar of the inner electrode conductor wirecomprising a drawn-filled tube filled with an inner core of a first typeof material, the at least one wire filar of the inner electrodeconductor wire wound to have a first pitch; a helically-shaped outerelectrode conductor wire disposed about and spaced radially apart fromthe inner electrode conductor wire, the outer electrode conductor wirecomprising at least one radially wound wire filar, each wire filar ofthe at least one wire filar of the outer electrode conductor wirecomprising a drawn-filled tube filled with an inner core of a secondtype of material different than the first type of material, the at leastone radially wound wire filar of the outer electrode conductor wirewound to have a second pitch that is less than the first pitch; at leastone insulation layer disposed radially about the inner electrodeconductor wire; and wherein the outer electrode conductor wire isconfigured to have an inductance sufficient to dissipate electromagneticenergy received on the lead from magnetic resonance imaging procedures,the inductance of the outer electrode conductor wire based at least inpart on the second pitch.
 10. The medical lead of claim 9, wherein eachwire filar of the inner electrode conductor wire has a wire diameter ofbetween about 0.001 inches to 0.004 inches.
 11. The medical lead ofclaim 9, wherein the inner electrode conductor wire has an outerdiameter of between about 0.020 inches to 0.028 inches.
 12. The medicallead of claim 9, wherein the resistance of the second type of materialis less than the resistance of the first type of type of material. 13.The medical lead of claim 9, wherein the outer electrode conductor wirehas a wire diameter of between about 0.001 inches to 0.006 inches. 14.The medical lead of claim 9, wherein the outer electrode conductor wirehas an outer diameter of between about 0.051 inches to 0.068 inches. 15.The medical lead of claim 9, wherein a wire diameter of the outerelectrode conductor wire is different than a wire diameter of the innerelectrode conductor wire.
 16. The medical lead of claim 9, wherein theat least one insulation layer comprises: a first insulation layerdisposed about the outer electrode conductor wire; and a secondinsulation layer interposed between the outer electrode conductor wireand the inner electrode conductor wire.
 17. An implantable medical lead,comprising: a helically-shaped inner electrode conductor wire disposedwithin an interior lumen of the lead, the inner electrode conductor wirecomprising a drawn-filled tube including a plurality of co-radiallywound wire filars each including an outer tubular layer that is filledwith an inner core of a first type of material having a firstresistance, the plurality of wire filars of the inner electrodeconductor wire wound to have a first pitch; a helically-shaped outerelectrode conductor wire disposed about and spaced radially apart fromthe inner electrode conductor wire, the outer electrode conductor wirecomprising a drawn-filled tube including a plurality of co-radiallywound wire filars each including an outer tubular layer that is filledwith an inner core of a second type of material that has a resistancethat is less than the resistance of the first type of material, theplurality of wire filars of the outer electrode conductor wire wound tohave a second pitch that is less than the first pitch; at least oneinsulation layer disposed radially about the inner electrode conductorwire; and wherein the outer electrode conductor wire is configured tohave an inductance sufficient to dissipate electromagnetic energyreceived on the lead from magnetic resonance imaging procedures, theinductance of the outer electrode conductor wire based at least in parton the second pitch.
 18. The medical lead of claim 17, wherein the firsttype of material has a silver content by cross-sectional area lower than28% and the second type of material has a silver content bycross-sectional area of about 28%.
 19. The medical lead of claim 17,wherein a wire diameter of the outer electrode conductor wire isdifferent than a wire diameter of the inner electrode conductor wire.20. The medical lead of claim 17, wherein the at least one insulationlayer comprises: a first insulation layer disposed about the outerelectrode conductor wire; and a second insulation layer interposedbetween the outer electrode conductor wire and the inner electrodeconductor wire.